[R] ability estimate with GRM of IRT

[SHEN,FENG]

Hi my friends,

I have an issue with ability estimates when running GRM of IRT.  I 
have responses from 242 subjects but got 183 ability estimates.  
Below is what I did to get the estimates.

1) I have a csv file "P1.csv" and I imported it into R and loaded 
the "ltm" package by doing:
p1<-read.table("P1.csv",header=TRUE,sep=",")
library(ltm)

2) I created a subset that included columns 2 to 9 for the 
analysis by doing:
s1<-p1[,2:9]

3) I converted the subset into data.frame format by doing:
s1df=data.frame(s1)

4) I checked the descriptive stats for the s1df by doing:
ds1df=descript(s1df)
ds1df

And it was confirmed that 242 subjects' responses were imported.

5)I ran GRM on the s1df dataset by doing:
grm<-grm(s1df, Hessian=T)
grm

6)Finally, I ran the ability estimated by doing:
aes1=factor.scores(grm)
aes1

And I got 183 factor-scores for observed response patterns.  
Besides, the 183 estimates are not for the first 183 subjects of 
the 242 because the response patterns of No 1 on the factor-scores 
list do not match those of No 1 in the s1 dataset and the same 
thing holds true for the rest of the response patterns.

Could you help me find out what the problem is?

Many thanks in advance!

Feng